<!DOCTYPE html>
<html lang="en">
    <head>
        <meta charset="UTF-8" />
        <meta http-equiv="X-UA-Compatible" content="IE=edge" />
        <meta name="viewport" content="width=device-width, initial-scale=1.0" />
        <title>Document</title>
    </head>
    <body>
        <script>
            let arr = [1, 2, 3, 4, 3, 2, 1];
            function unique(arr) {
                let newArr = [];
                for (let i = 0; i < arr.length; i++) {
                    if (!newArr.includes(arr[i])) {
                        newArr.push(arr[i]);
                    }
                }
                return newArr;
            }
            console.log(unique(arr)); // [1,2,3,4]
        </script>
    </body>
</html>
